This invention relates to a PLC (programmable logic controller).
The PLC includes an I/O module to which an external device is electrically connected, a CPU module which executes an execution program for controlling the external devices through the I/O module, etc. The PLC permits a user to freely program the execution program. However, in order to program the execution program, considerable knowledge on the PLC is required. Namely, in order that the user creates the execution program, considerable knowledge and labor are required.
Now, as an example of the PLC, a safety PLC is cited. In the safety PLC, advanced knowledge on safety control as well as the knowledge on the PLC is required. The safety PLC includes an I/O module to which a safety device such as an emergency stop switch or a light curtain is connected, a CPU module which executes a safety program (corresponding to the above execution program) for controlling the safety devices through the I/O module, etc.
The safety PLC employed in e.g. a production line, when the production line falls in a dangerous state, on the basis of a predetermined fail-safe operation, stops a part or entirety of the production line. Now, the dangerous state refers to a state when the emergency stop switch is pushed, a state when a sensor such as the light curtain detects entry of a man. In short, the safety PLC is employed in the production line or the like so that a worker can work more safely therein.
Further, the safety PLC must be suited to the safety standards such as the European Safety Standard or International Safety Standard, and so must be examined and certified by a certifying institution.
Now, the safety PLC includes the I/O module to which the safety device is connected and the CPU module. The I/O module and a packaged intrinsic control program correspond to each other in a one-to-one relationship. The control program is incorporated in the CPU module and serves as a safety program executed as the safety PLC.
In such a safety program, in order to program the safety program, as described above, advanced expertise and rich experience on the safety standard and safety control are required. Thus, a longer time and higher production cost have been needed as compared with the case of creating a general execution program.
In order to avoid such an inconvenience, JP-A-2007-25736 previously filed by the applicant of this application proposes a safety PLC in which the safety program is created by only connecting the I/O module to the CPU module. Specifically, a fixed function block suited to the safety standard, which is prepared previously, is connected on the basis of a connecting order of the I/O module. Thus, the safety PLC has become capable of being used with no advanced expertise.
In this way, in the PLC, since the execution program (safety program) is automatically created, even the user who does not have expertise has become capable of using the PLC without consuming time and cost.
However, in the above PLC, the execution program automatically created was black-boxed so that it could not be edited or even seen. The intrinsic control program serving as the execution program (function block in JP-A-2007-25736) also was the packaged fixed program and so black-boxed.
Namely, for the demand that a part of the execution program automatically created is desired to be edited, it could not be edited or even seen. Therefore, for example, even where there were slight changes in the production line, changes in hardware such as recombination of the I/O modules and changes in the external devices were needed. Namely, since the execution program automatically created cannot be edited, there are fears of generating the problems such as making wiring troublesome, an increase in a device establishing space, cost increase, etc.